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We have demonstrated a novel approach to construct a cascaded Mach-Zehnder interferometer (MZI) based on local crystallization of sapphire-derived fiber (SDF). When the SDF is spliced to standard single mode fibers, the crystallization of SDF occurs near the spliced points. Such crystallized points in SDF could excite and recombine two core modes, which leads to an in-fiber MZI. Moreover, by applying an arc discharge to the SDF, the crystallization could create a cascaded MZI with a temperature-insensitive feature, showing great potential in industrial sensing applications where the required sensors can endure high temperature as well as exhibit low cross-sensitivity from temperature.
© 2017 Optical Society of America
1. Introduction
The Mach-Zehnder interferometer (MZI) presenting filter effect has attracted considerable attention in the application of various fields including physical, biological, chemical sensing and telecommunication [1–6]. Especially, all in-line fiber MZI is of great interest because of the advantages including small size, flexible design, fast response, high sensitivity, and good stability. Most of the in-line fiber MZI is based on core-cladding modes interference by structural modification, such as optical fiber taper [7], misaligned spliced joint [8] and long-period fiber grating [9]. Such structural distribution can couple the incident core mode into one or several cladding modes. However, due to the relative large difference between the interference paths induced by ambient temperature variation, the MZIs suffer from the temperature cross-sensitivity problem, which leads to a poor accuracy in optical fiber sensing applications and the instability of filter channel in optical communication system. At present, the general methods to overcome the limitation by temperature are to design compound sensing structures or adopt temperature compensation technique. Y. Cao et al [10] have demonstrated a typical compound sensing structure based on a core-offset MZI and a long-period fiber grating (LPFG). Utilizing the twin sensors, the simultaneous measurement of the temperature and external RI is demonstrated with high sensitivity of 0.0607 nm/°C and −18.025 nm/RIU. The temperature and external RI are calculated through the sensitive matrix of the system, which undoubtedly increases the difficulty in signal demodulation. The temperature compensation technique utilizes the dependence of the MZI on the thermal-optic and thermal expansion effects. W. Ding et al. [11] have investigated a ceramic package method to compensate a PCF-based MZI component. The temperature sensitivity can be reached to 1 pm/°C. But it only shows a good composition result within a relatively low temperature range of 20~160 °C.
On the other hand, the sapphire-derived fiber (SDF) is an all-glass fiber with an aluminosilicate core, which has a number of merits due to a series of useful properties, such as lower Brillouin gain coefficient, photoblastic constants, and high temperature stability [12]. In 2012, the SDF was firstly proposed and reported for the extra high alumina dopant concentration to silica [13]. With the high alumina dopant concentration, it demonstrates very small Brillion gain coefficient, showing potential applications in nonlinear optical fields. In 2014, the SDF was used for fiber Bragg grating (FBG) inscription by means of femtosecond-laser pulses. The successfully inscribed temperature-stable FBG in SDF can work under temperature up to 950 °C for more than 24 hours without degradation in reflectivity [14].
In this paper, we have proposed and demonstrated a novel in-line fiber cascaded Mach-Zehnder interferometer (MZI) based on the sapphire-derived fiber (SDF). Attributed to the high alumina dopant concentration, the core of SDF exhibits high refractive index difference of ~0.06 and presents crystallization behavior under reheating and cooling treatment [15]. Utilizing the crystallization feature, the mullite micro-size particles can be generated locally as the SDF is treated using electrical arc discharge. With the local crystallization-based refractive index (RI) modification, the higher-order core mode can be excited in the SDF. Therefore, an in-fiber MZI can be constructed with modal interference. In addition, the fundamental core mode and excited higher-order core mode experience quite similar interference paths due to the fiber core with high refractive index, which is different from general modal interference between core and cladding modes. As a result, the filtering spectrum of the MZI presents good temperature stability of ~4.6 pm/°C from room temperature to 900 °C after annealing. The developed cascaded MZIs based on SDF show great potential in the application where the required sensors can endure high temperature as well as exhibit low cross-sensitivity from temperature, for example, the industrial sensing in airspace engine, oil and gas drilling, etc.
2. Proposal for the in-line fiber cascaded Mach-Zehnder interferometers
Figure 1 shows the schematic diagram of our proposed cascaded MZIs which are constructed by three crystallization regions with refractive index modification, leading to a non-uniform distribution of fiber along longitudinal direction for the core-core modes coupling. The SDF with a length of L is connected to lead-in and lead-out single mode fibers (SMFs) with a crystallization region near the spliced points M1 and M3. Then, a small dose of extra arc discharge can be applied over the SDF at position M2, forming crystallization region, which has a distance of L` from M1. At the spliced point M1, the higher order core mode is excited with RI modification by crystallization. As it travels to position M2, it could be partly coupled to the core mode and excited again, then at the spliced point M3, the two modes including new excited component and the ones not participating in the interference at M3 are totally coupled at the input of the lead-out SMF and interfere with each other. Therefore, using this configuration, an inline cascaded MZI with three interference cavity lengths of L`, L –L` and L can be formed.
Fig. 1 Schematic diagram of light propagating along the cascaded Mach-Zehnder interferometer in sapphire-derived fiber (SDF); single mode fiber (SMF)
3. Fabrication and characterization of cascaded Mach-Zehnder interferometers
In the experiment, the SDF is a special fiber with high concentration of alumina to silica in the core, which is fabricated by using sapphire rod as an initial core phase in the resultant fiber [16]. The concentration of alumina is up to 32 mol.% in the alumina-silica (Al2O3-SiO2) composite core. The fabricated SDF was tested by using a refractive index profiler (S14). Its diameter and core diameter are 125 µm and 18 µm, respectively. And the maximum refractive index difference is 0.06 between the core and cladding. At first, the SDF and standard SMFs are well cleaved, followed by the fusion splicing through a commercial optical fiber splicer (FITEL-s178) in a manual mode. We set the intensity and time duration of arc discharge to be 16.6 mA and 3s. Figures 2(a) and 2(c) show the microscopic images of fiber near the splicing points where dark regions are found. Moreover, an extra arc discharge is applied to SDF at position M2. The arc discharge intensity and duration are set to be 13.3 mA and 500 ms, which will form two dark regions separated by ~600 μm as shown in Fig. 2(b). In the experiment, the dark region can be well controlled by the location, duration and intensity of arc discharge.
Fig. 2 Microscopic images of the SDF with crystallization near splicing points (a) M1 and (c) M3, and arc discharge at position M2
The formation of dark region is believed to originate from the rapid crystallization behavior of the high concentration alumina-silica composite core during the reheating and cooling process generated by arc discharge [15]. Based on the principle of spinodal nuleation mechanism [17], the crystallization of mullite within the high-alumina-dispersed glass phase may occur in the region of metastable immiscibility [18]. When the arc discharge temperature is in the crystallization temperature range, according to the Al2O3-SiO2 phase diagram [19], the separation of fine co-connective glass phase as well as the crystallization of mullite occurs and the core presents a dark region, as shown in Figs. 2(a)-2(c). Attributed to the crystallization in the core treated by the arc discharge, the material in fiber core is modified while the RI can be modified locally. Such crystallization can achieve a maximum RI modification of ~0.015 measured by a three-dimension RI profile system based on the digital holographic tomography technology [20].
The transmission spectrum of the SDF-based MZI that only consists of two spliced points is initially characterized. The spectrum is modulated quasi-sinusoidally, and the fringe spacing will increase as the SDF length L decreases as shown in Fig. 3(a). The spectra show little inhomogeneous, thus there might be more than two modes involving in an inhomogeneous interference pattern.
Fig. 3 (a) Transmission spectra of SDF with different interference length (the spectra are offset by 10 dB for comparison), (b) the corresponding spatial frequency spectra with different interference length.
In order to investigate how many modes involve in the interference, the wavelength spectra are Fourier transformed to get the spatial frequency as shown in Fig. 3(b). The relationship between the spatial frequency and the interferometer length (L) as well as the differential modal group index is given by [21, 22],
where λ is the center wavelength, Δneff represents the effective refractive group index difference between different modes. The results are shown in Fig. 3(b). Obviously, one peak dominates in the interference for each spectrum. In the experiment, when we drop the index matching oil on the developed MZI in order to remove the cladding modes, the interference spectra still maintain the same interference pattern as before, indicating that a higher order core mode interferes with the fundamental core mode that forms the main interference pattern. In order to figure out the number of modes, we have theoretically investigated the mode properties of SDF based on the finite-element method [23]. The participated modes in interference are found to be LP01 and LP11 modes, and Δneff is calculated to be 2.02 × 10−3. In the experiment, according to the spatial frequency shown in Fig. 3(b), Δneff between the modes participating in the interference is estimated to be 2.16 × 10−3. The experimental measurement of Δneff is well consistent with the simulated result, confirming that the excited modes are LP01 and LP11 modes.To find where the excitation of LP11 mode takes place, the experimental investigation is further conducted. The mechanism is that the excitation of LP11 mode could induce an excess loss because LP11 mode will be filtered out by the following standard SMF. Firstly, the crystallization region of SDF is removed during fiber splicing by controlling the intensity and the duration of arc discharge. The length of SDF between lead-in/lead-out SMFs is kept at ~655 μm to minimize the attenuation from the fiber itself. An attenuation of ~0.45 dB is found for the scheme of SMF-SDF-SMF at wavelength of 1550 nm, which is mainly attributed to the two spliced points. Secondly, the crystallization region is introduced in SDF during fiber splicing. The attenuation is raised up to ~5 dB, indicating that the crystallization of SDF mainly contributes to the excitation of LP11 mode and thereby the performance of the MZI.
After the MZI is constructed by the two spliced points with lead-SMFs, the extra arc discharge is applied to the SDF at position M2. The first sample is with a SDF length of L = 2 cm and at a discharge position of L` = L/2. The transmission spectra with and without extra arc discharge are compared in Fig. 4(a). After the extra arc discharge, a small side dip among the adjacent main dip is filtered out and the separation between main dips is enlarged by two times. In order to get more information from the spectral evolution, the fast Fourier transform of the wavelength spectrum is taken as described in Eq. (1). The results are shown in Fig. 4(b). Without extra arc discharge at position M2, the interference spectrum is sinusoidal and only one dominant peak at 0.018 nm−1 in frequency is observed. However, after one extra arc discharge, another peak at 0.009 nm−1 emerges and the comparable amplitudes for the two periods could be achieved. Figures 4(c) and 4(d) show the wavelength spectra of MZIs with L` = L/3 and L = 2 cm. It can be seen that one third spatial frequency at 0.006 nm−1 and two-thirds frequency at 0.012 nm−1 have been produced. Their amplitudes are comparable to the one at 0.018 nm−1. Figures 4(e) and 4(f) refer to the MZIs with L` = L/4 and L = 2.1 cm, where we find a quarter frequency at 0.005 nm−1 and three-fourths frequency at 0.015 nm−1 compared to frequency 0.020 nm−1. Indeed, there should be always three spatial frequencies when put an arc point at M2. Besides the two dominant frequencies caused by the modal interference with a cavity length of L-L` and L`, a third peak of spatial frequency is caused by a cavity length of L. Two frequencies are observed for L` = L/2 because the two frequencies before and after the arc point are overlapping with each other.
Fig. 4 Transmission spectra of SDF-based MZI without (short dash) and with arc discharge (solid line) at various positions (a) L = 2 cm, L` = L/2; (c) L = 2 cm, L` = L/3; (e) L = 2 cm, L` = L/4; (b), (d) and (f) refer to the corresponding spatial frequency spectra.
4. Cascaded Mach-Zehnder interferometers in sapphire-derived fiber for temperature-insensitive filters
Figure 5 shows the experimental setup for temperature measurement, consisting of an amplified spontaneous emission (ASE) broadband light source, a tube furnace (resolution ± 1 °C) for temperature control, and an optical spectrum analyzer (OSA, Yokogawa AQ6370) for recording transmission spectrum. For high temperature test, we use the developed device with SDF length of L = 2 cm and the discharge position at L` = L/2, because its spectrum shows a large resonant dip near 1560 nm. The cascaded MZIs are kept straight during the whole experiment. The resonant dip is chosen as an indicator for temperature measurement. During the temperature test, the furnace is gradually heated to 900 °C with a step of 100 °C, and stayed for 10 min at each step. In order to remove the burnt fiber coating induced effects as well as examine whether the dopant diffusion could seriously deteriorate the transmission spectrum, the furnace temperature is maintained at 900 °C for 1 hour before cooling down.
Fig. 5 Schematic diagram of temperature measurement system for SDF-based cascaded MZIs: amplified spontaneous emission (ASE) light source; optical spectral analyzer (OSA); single mode fiber (SMF); Mach-Zehnder interferometer (MZI).
Figure 6 shows the spectral evolution of cascaded MZIs exposed to different temperature values. By increasing temperature, the resonant dip slightly fluctuates around 1560 nm. When the temperature reaches > 800 °C, the wavelength of dip sharply shifts towards a shorter wavelength by 8.36 nm as shown in Fig. 6(a). This phenomenon is explained due to the relaxation of internal stresses frozen in the fiber during drawing [24]. With instantaneously releasing the frozen stress during 900 °C for 1 hour, the dip will sharply change to a new equilibrium value and then gradually stabilize over time. Therefore, in the first cooling and second warming and cooling process, the interference dip remained a good repeatability to temperatures cycles as shown in Fig. 6(b). From the experimental results shown in Figs. 6(a) and 6(b), we also find that the temperature has an effect on the attenuation of the resonant dip. When the temperature increases, the attenuation of the resonant dip gradually increases. When we reduce the temperature, the attenuation of the dip decreases and can be reversed back to the similar level at room temperature. The small difference in attenuation at room temperature before and after annealing is believed to be the stress relaxation [24]. Such reversible attenuation indicates that the change of temperature can alter the coupling coefficient of modes participating in the interference [25]. Since the crystallization of mullite has different coefficient of thermal expansion from the alumina-silica composite core [26], the geometry of SDF can be altered when the temperature is changed. Therefore, the attenuation of resonant dip varies when temperature is changed. After several heating and cooling temperature cycles, the interference dip remains stable as shown in Fig. 6(c). The sensitivity in terms of the wavelength shift versus the temperature change is calculated to be ~4.6 pm/°C, which is less than 12.3 pm/°C reported by previous work on polarization-maintaining photonic crystal fiber in high temperature [27].
Fig. 6 Interference dip of the cascaded MZI under different heating and cooling temperature cycles (over the range of 17 °C to 900 °C) at (a) the first heating process, (b) the second cooling process. (c) Wavelength responses of MZI under several heating and temperature cycles.
The temperature response of cascaded MZIs with the L` away from L/2 is also investigated. Two samples of cascaded MZIs with L` = L/3 and L` = L/4 are explored as shown in Figs. 4(c) and 4(e). When the temperature varies from room temperature to 900°C, the temperature sensitives of the above MZIs are calculated to be ~5.1 pm/°C and ~3.7 pm/°C, respectively. Such sensitivities are at the same level to the sensitivity of the MZI with L` = L/2, which is ~4.6 pm/°C. Previous reports have theoretically shown that the temperature response of fiber-based MZIs is mainly determined by the material and refractive index of the fiber, rather than the fiber length [28]. The extension of SDF length actually increases the interference length, so the extension of SDF length almost does not affect the temperature response of the MZIs. Our experimental results confirm that the temperature sensitivity of MZIs is weakly correlated with the location of arc point. The small difference in the measured response sensitivities of the MZIs might be caused by the optical spectrum analyzer with a resolution of 0.2 nm as well as the imperfect consistence during manufacturing MZIs. Therefore, our developed cascaded MZIs show a good high temperature-insensitive feature to overcome the temperature cross-sensitivity limitation. Because of the large refractive index difference of 0.06 between the fiber core and cladding in SDF, the propagation of mode is easily confined in the fiber core. Simultaneously, owing to the fact that the guided core modes involve in the interference, they have very similar thermal coefficients which leads to the effective refractive index change similarly with the change of temperature. Thus the interference between core-core modes in SDF shows low temperature sensitivity. After annealing treatment, we can obtain a temperature-insensitive bandpass filter to high temperature level. Note that a conventional GeO doped fiber has a high index contrast can also achieve a sensor that is temperature insensitive, and the related work has been reported [29]. Compared with the current temperature-insensitive sensors based on solid-core fibers [29, 30], our proposed sensor based on SDF have much smaller temperature sensitivity that can be can be utilized in the targeted application. Moreover, the crystallization in SDF through annealing provides a feasible way to modify the RI of SDF, which shows potential applications to achieve functional devices [15].
5. Conclusion
We have demonstrated a novel cascaded MZI by applying arc discharge to the SDF. Utilizing the crystallization feature, the mullite micro-size particles can be generated locally when the SDF is treated by using electrical arc discharge. With the local crystallization-based refractive index modification, the higher-order core mode can be excited in the SDF. Therefore, an in-fiber MZI can be constructed with modal interference. As the fundamental core mode is coupled with the excited higher-order core mode, the filtering spectrum of the MZIs presents good temperature stability of 4.6 pm/°C from room temperature to 900 °C after annealing. The developed cascaded MZIs based on SDF show great potential in the applications where the required sensors can endure high temperature as well as exhibit low cross-sensitivity from temperature. The targeted application includes the industrial sensing in airspace engine, oil and gas drilling, etc.
Funding
National Natural Science Foundation of China (Grants 61422507, 61635006, and 61605108); National Key Research and Development Program of China (2016YFF0100603).
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